For power lithium-ion batteries, the indicators we pay most attention to are energy density and power density, which are related to the vehicle's range, while power density is related to the dynamic performance of electric vehicles. How to improve the rate performance of lithium ion battery designers have their own unique insights, small make up here to talk about some of my ideas to improve the rate performance of lithium ion battery, I hope to be able to cast a brick to attract others.
1. Material selection
Generally speaking, the improvement of power battery rate performance is mainly from the choice of materials. Comparison of ionic and electronic conductivity between common high nickel ternary materials and traditional lithium cobalate materials [1], at room temperature of 20℃, the lowest electronic conductivity of LCO material is only 5x10-8s /cm, while the electronic conductivity of NCM111 material can reach 2.2x10-6s /cm. With the further increase of nickel content, The electronic conductivity of ternary materials is also significantly improved. The electronic conductivity of NCM8111 material is up to 4.1x10-3s /cm, and the ionic conductivity also shows the same trend. The ionic conductivity of LCO material is only 2.3x10-7s /cm at 20℃. However, the ionic conductivity of NCM111 is 3.2x10-6s /cm, NCM532 is 1.7x10-3s /cm, NCM622 is 3.4x10-3s /cm, and NCM811 is even 6.3 x10-3s /cm. Therefore, whether from the perspective of electronic conductivity or ionic conductivity, Especially high nickel ternary material or NCA is more suitable for multiplying power type lithium ion battery, of course, in addition to the intrinsic characteristics of the material, its performance is affected by the morphology of multiple factors such as ratio, such as small particles of material surface area is bigger, Li + inside the particle diffusion distance is shorter, so in theory will have better ratio of performance.
There are many kinds of anode materials, such as small particles of intermediate graphite materials, which have good performance in the rate performance. S.R. Sivakkumar, J.Y. Nerkar,A.G. Pandolfo, Energy Technology Department of CSIRO [5] evaluated graphite materials of different types and particle sizes and showed that the smaller the particle size of graphite materials, the higher the rate performance. Reducing the coating thickness of graphite can also improve the rate performance of graphite anode. However, the reduction of particle size also brings a series of problems, such as the reduction of reversible capacity and compaction density. Meanwhile, the study also shows that although the above measures can improve the discharge rate performance of graphite anode, but it is difficult to effectively improve the charging rate performance of graphite anode.
Li4Ti5O12 material itself has a high Diffusion coefficient of Li+ (10-16-10-15m2/S) [2]. At the same time, due to the low conductivity of lithium titanate battery material, nano particles are often made in production, which further increases the active area and reduces the diffusion distance of Li+. Therefore, lithium titanate battery has excellent rate performance and can achieve fast charging, which is also the reason why Dong Ming-zhu sees Zhongyinlong. However, the voltage platform of lithium titanate material is 1.55V, and the theoretical reversible capacity is 170mAh/g, resulting in low specific energy of battery, which seriously affects the range of electric vehicles. This is also the root cause of the recent crisis in Silver Long, is the so-called into also xiao, also xiao. In order to solve the problems of the lithium titanate, while retaining its advantage of high ratio performance of scientific research workers have done a lot of effort, Toshiba [3] the development of titanium niobium oxides NTO new anode materials, the material of the irreversible capacity is 341 mah/g is far higher than that of LTO materials, close to graphite material, but with the advantage of high density, At the same time, the material also retains the characteristics of fast charging. It only takes 6 minutes to charge from 0%SoC to 90%SoC, which almost perfectly meets the needs of electric vehicles.
In a recent article published in Nature, Kent J.Griffith [4] describes the latest research from the University of Cambridge: Nb16W5O55 and Nb18W16O93 materials, which have reversible capacities of more than 200mAh/g at C/5 ratio, and the diffusion coefficients of Li+ in the two materials reach 10-13-10-12m2/S, much higher than LTO(10-16-10-15m2/S). Can therefore micron grade achieve excellent performance ratio on particle size, larger particles not only reduces the active material/electrolyte interface area, reduce the incidence of adverse events, but also greatly increase the material compaction density, so the two materials in terms of unit volume capacity exceptional, crush all the cathode material.
2. Formula optimization
Decided to lithium ion battery ratio of another is the key to the performance of the battery formula design, in the lithium ion battery within "ionic conduction" and "electronic conductive" two conductive forms, including ion conductive mainly include Li + in the electrolyte and electrode internal pore and the spread of the active material internal electronic conductive mainly between the active material particles, The electronic conductivity can also be divided into "short-range conductivity" and "long-range conductivity". For example, the conductive agent represented by carbon black is mainly responsible for short-range conductivity, while the conductive agent represented by carbon fiber and carbon nanotubes is mainly responsible for long-range conductivity. The multiplier performance of lithium-ion batteries is a comprehensive reflection of several forms of conductivity. The research of Samantha L. Morelly et al. [6] from Drexel University in the United States shows that the key factor affecting the multiplier performance of lithium-ion batteries is not the process of "ion diffusion" as commonly thought, but more dependent on electronic conductivity. For example, the power performance of the electrode with 3% carbon black is obviously better than that of the electrode with 2.5%. However, according to the "ion transfer" limitation theory, more carbon black means a more tortuous Li+ diffusion channel, which will reduce the power performance of the lithium ion battery. Meanwhile, the study shows that compared with the long-term conductivity, The short-range conductivity provided by carbon black adsorbed on the surface of NCM particles plays a greater role in improving the rate performance of lithium-ion batteries.
It is not difficult simply to achieve high rate performance, but the difficulty lies in the balance between rate performance and energy density. Generally speaking, the rate performance and energy density are contradictory, and it is very difficult to find a balance between them. Kazuaki Kisu et al., Tokyo Agricultural and Technical University, Japan [7] obtained the optimal combination of coating thickness and compaction density (70um and 2.9g/cm3) by analyzing the impedance of NCM electrodes with different coating thickness and compaction density. If the compaction density is too high, the electrode porosity will decrease sharply, leading to the increase of ion diffusion impedance. Low compacting density will lead to an increase in contact impedance. Therefore, only an appropriate compacting density can ensure the excellent rate performance of lithium-ion batteries and also take into account the characteristics of high energy density.
3. Selection of battery structure
For times permeance cells in the process of how to control the discharge temperature is also a very important question, during the process of high discharge current lithium ion battery can produce a lot of heat, heat in lithium ion battery internal accumulation will lead to a rise in temperature, large temperature gradient, so the lithium ion battery internal failure down inconsistent, affect the service life of lithium ion batteries. How to choose an appropriate structure becomes particularly important. Stephan Kosch et al., Technical University of Munich, Germany [8] studied the influence of shape and position of pole lug of lithium ion battery on thermal characteristics of large-size lithium ion battery through two-dimensional electro-thermal polarization model. Very ear width and the thickness of the collection of fluid for lithium ion battery in the temperature distribution in the process of discharge, the narrower the pole ear, the thinner the battery set fluid temperature distribution inside the nonuniformity, the greater the also found that when the battery pole ear on the ends of the battery when can effectively reduce the discharge of the battery internal temperature non-uniformity in the process of.
By selecting appropriate material, formula and structure, the internal impedance and polarization of lithium ion battery can be reduced, the temperature inhomogeneity can be reduced, and the efficiency of the battery can be effectively improved. Increase rate performance is a comprehensive project, need to consider from multiple factors, xiaobian introduction is only a drop in the bucket, knowledge limits are unavoidable omissions, I hope friends criticism and correction, put forward their own views.
